Virus-Induced Polykaryocytosis And The Mechanism Of Cell Fusion

The development of cytopathic effects (CPE) in cells infected with viruses is the most obvious means by which their presence is detected. The nature of the CPE produced by viruses is highly characteristic and provides an important criterion for their assay and classification. Polykaryocytosis, which is the formation of multinucleate cells, variously termed polykaryocytes, syncytia or giant cells, is one such characteristic type of cellular response to virus infection. This chapter discusses the evidence to show that most virus-induced polykaryocytes are formed by cell fusion. However, viruses represent only one of the many agents that can induce polykaryocytosis and cell fusion. The chapter discusses on virus-induced cell fusion and deals with the broader aspects of the process of cell fusion. It discusses the factors that significantly influence polykaryocyte formation: the cell, the virus, and the culture environment. The polykaryocytes induced by viruses are in most cases formed by the fusion of previously separate single cells. In a few cases, polykaryocytes of limited size can also be formed from single cells, either by repeated karyokinesis without cytokinesis or by multiple amitotic nuclear division. It is concluded that virus-induced cell fusion does not require that infected cells fuse only with uninfected cells. Fusion can occur also between infected cells and even between polykaryocytes. The polykaryocytes produced by cell fusion represent true multinucleate cells in which the cytoplasm is not subdivided by membranes.